The present invention relates to process control transmitters used to measure process variables in industrial processing plants. More particularly, the present invention relates to field devices with wireless transceivers powered by an external common DC bus for power supply or by an existing power circuit.
In industrial settings, control systems monitor and control inventories, industrial and chemical processes, and the like. Typically, the control system perform these functions using field devices distributed at key locations in the industrial process and coupled to the control circuitry in the control room by a process control loop. The term “field device” refers to any device that performs a function in a distributed control system, including all devices currently known in the measurement and control art.
Generally, each field device includes a transducer. A transducer is understood to mean either a device that generates an output signal based on a physical input or that generates a physical output based on an input signal. Typically, a transducer transforms an input into an output having a different form. For example, a loudspeaker is a transducer that transforms electrical signals into sound energy. Types of transducers include various analytical equipment, pressure sensors, thermistors, thermocouples, strain gauges, flow transmitters, positioners, actuators, solenoids, indicator lights, and the like.
Typically, each field device also includes a transmitter that boosts the transducer's signal in a standard format. Such transmitters generally communicate with the control room via the process control loop. Typically, the process control loop delivers a regulated current and/or voltage for powering the field devices. Additionally, the process control loop may carry encoded signals.
Traditionally, analog field devices have been connected to the control room by two-wire twisted-pair current loops, with each device connected to the control room by a single two-wire twisted pair loop. Typically, a voltage differential is maintained between the two wires within a range of voltages from 12-45 volts for analog mode and 9-50 volts for digital mode. An analog field device transmits a signal to the control room by modulating the current running through the current loop to a current proportional to the sensed process variable. An analog field device that performs an action under the control of the control room is controlled by the magnitude of the current through the loop, which is modulated by the ports of the process subsystem under the control of the control room.
Discrete or digital field devices respond to a binary signals and transmit binary information. Typically, discrete devices operate with a 5-30 volt signal (AC or DC), a 120 or 240 volt AC signal, delivered by the same or similar two-wire twisted pair loops. Of course, a discrete device may be designed to operate according to any electrical specification required by the control environment.
Generally, in industrial plants, the individual field devices are wired to a junction box, and from there to the control room or to marshaling racks through home run cables. Since cabling distances from the field device to the junction box are relatively short, the bulk of the cabling cost is in the home run cable. HART® is a well established standard but control systems in general do not support HART® multidrop configurations; therefore, there is little savings from wiring using the HART® protocol. The few control systems supporting HART® generally have limited access to device diagnostics and do not use the digital information for control due to speed limitations.
Since communications and power typically are delivered over the same wires, various properties must be taken into account in order to have a successful installation, such as proper shielding against noise, low ripple power supplies, appropriate line and power impedances, wire length and properties, impedance, terminations, and the like. Using the same pair of wires for communication and power also makes power regulation of the device more complicated. Simple low pass filters cannot be used to remove noise from the power signal because a “notch” at the communications frequency must be allowed to pass. Specifically, the low pass filter has a tendency to “refine” the load current, thereby reducing ripples or notches in the AC communication signal. It is important to note that the transmitters are basically shunt regulators that shunt between 4 and 20 mA in analog transmitters. In order to shunt on the low end (i.e. at 4 mA), the transmitter must operate at a power of less then 4 mA.
With the advent of low power wireless communications, many new network topologies can be imagined. However, power constraints of wireless transmitters typically limit process control networks to the traditional wired topology. A truly wireless field device is one that contains its own source of power, such as a battery, solar power, fuel cell, an energy scavenging power source, and the like, and is not constrained by traditional wired configurations.
However, such wireless transmitters suffer from the basic problem of low available power, which tends to limit the utility of such field devices. Specifically, the low available power forces the use of low data rates to help conserve energy, and/or requires frequent, periodic replacement of the power source. Transmission power also restrains the transmitting distance.